Flat-panel displays are being developed which utilize liquid crystals or electroluminescent materials to produce high quality images. These displays are expected to supplant cathode ray tube (CRT) technology and provide a more highly defined television picture or computer monitor image. The most promising route to large scale high quality liquid crystal displays (LCDs), for example, is the active-matrix approach in which thin-film transistors (TFTs) are co-located with LCD pixels. The primary advantage of the active matrix approach using TFTs is the elimination of cross-talk between pixels, and the excellent grey scale that can be attained with TFT-compatible LCDs.
Flat panel displays employing LCDs generally include five different layers: a white light source, a first polarizing filter that is mounted on one side of a circuit panel on which the TFTs are arrayed to form pixels, a filter plate containing at least three primary colors arranged into pixels, and finally a second polarizing filter. A volume between the circuit panel and the filter plate is filled with a liquid crystal material. This material will allow transmission of light in the material when an electric field is applied across the material between the circuit panel and a ground affixed to the filter plate. Thus, when a particular pixel of the display is turned on by the TFTs, the liquid crystal material rotates polarized light being transmitted through the material so that the light will pass through the second polarizing filter.
The primary approach to TFT formation over the large areas required for flat panel displays has involved the use of amorphous silicon, which has previously been developed for large-area photovoltaic devices. Although the TFT approach has proven to be feasible, the use of amorphous silicon compromises certain aspects of the panel performance. For example, amorphous silicon TFTs lack the frequency response needed for high performance displays due to the low electron mobility inherent in amorphous material. Thus the use of amorphous silicon limits display speed, and is also unsuitable for the fast logic needed to drive the display.
As the display resolution increases, the required clock rate to drive the pixels also increases. In addition, the advent of colored displays places additional speed requirements on the display panel. To produce a sequential color display, the display panel is triple scanned, once for each primary color. For example, to produce color frames at 20 Hz, the active matrix must be driven at a frequency of 60 Hz. In order to reduce flicker it is desirable to drive the active matrix at 180 Hz to produce a 60 Hz color image. At over 60 Hz, visible flicker is reduced.
Owing to the limitations of amorphous silicon, other alternative materials include polycrystalline silicon, or laser recrystallized silicon. These materials are limited as they use silicon that is already on glass, which generally restricts further circuit processing to low temperatures.
Integrated circuits for displays, such as, the above referred color sequential display, are becoming more and more complex. For example, the color sequential display is designed for displaying High Definition Television (HDTV) formats requiring a 1280-by-1024 pixel array with a pixel pitch, or the distance between lines connecting adjacent columns or rows of pixel electrodes, being in the range of 15-55 microns, and fabricated on a single five-inch wafer.
In accordance with the invention, the cost and complexity of high resolution displays is significantly reduced by fabricating multiple integrated displays of reduced size on a single wafer and then dicing the wafer to produce a plurality of display devices.
The displays are then assembled with appropriate magnifying optics to form a portable display system of low cost and reduced size. Included in the optics is a magnification system which compensates for the small image size by magnifying and projecting the image at an appropriate distance for viewing.
In preferred embodiments, the microdisplay, because of its small size and weight, can be used as a hand-held communication system such as a pager, a wireless mobile telephone, or alternatively, as a head-mounted display, video camcorder, digital camera or a card reader display system. The display can provide a visual display suitable for data, graphics or video and accommodate standard television or high definition television signals. The system can optionally include circuitry for cellular reception and transmission of facsimile communications, can be voice activated, can include a mouse operated function, provide Internet access, and can have a keyboard or touch pad for numeric or alphabetic entry. The system can have, such as in a card reader display system, a housing with a port or aperture to receive a card, and a card reader for reading information from the card and displaying the information on the micro-display.
The telephone or hand-held unit can be equipped with a camera or solid state imaging sensor so that images can be generated and transmitted to a remote location and/or viewed on the display. Also the telephone user can call to access a particular computer at a remote location, present the computer screen on the microdisplay, access specific files in the computer memory and download data from the file into a memory within the telephone or a modular memory and display unit connected to the telephone. The telephone can be connected to a local computer or display and the data from the file can be loaded into the local memory.
The video camcorder or digital camera has a microdisplay for a viewfinder. Either an image as seen through the lens or as previously recorded can be seen through the viewfinder, depending on what is selected.
In a preferred embodiment of the invention, a light emitting diode (LED) device is used to illuminate the display. For transmission displays the LED device operates as a backlight and can include a diffuser. An LED device can also be used as a light source for a reflective display in another preferred embodiment of the invention. The displays are preferably liquid crystal displays using a twisted nematic liquid crystal material. Consequently, controlling the time domain is not necessary to obtain grey scale.
For the purposes of this application, a microdisplay is defined as a display having at least 75,000 pixel electrodes and an active area of less than 160 mm2, where the active area of the display is the area of the active matrix circuit that generates an image, including all of the pixel electrodes but not including the driver electronics and the border area for bonding and sealing of the liquid crystal display. For example, the array can be at least 320xc3x97240, 640xc3x97480 or higher. A preferred embodiment of the microdisplay has an active area of 100 mm2 or less, and is preferably in the range between 5 mm2 and 80 mm2. The pixel pitch for these displays is in the range of 5-30 microns and preferably in the range between 5 and 18 microns. By utilizing pixel pitches of less than 18 microns smaller high resolution displays are now possible. For an embodiment utilizing a high definition format such as 1280xc3x971024, and utilizing a pixel pitch of 12 microns or less, the active area of the display is less than 200 mm2.
For displays of this size and resolution to be read by a user at distances of less than 10 inches (25.4 cm) there are specific lighting and magnification requirements. For a 0.25 inch (6.35 mm) diagonal display, for example, the LED device preferably includes a plurality of LEDS coupled to a diffuser. The lens used to magnify the display image has a field of view in the range of 10-60 degrees, and preferably at least about 16 degrees-22 degrees, an ERD in the range of about 25 mm-100 mm and an object distance of between about 1.5 and 5 feet (152.4 cm). A color field sequentially operated LED backlight system can use a plurality of LEDS with a two or four sided reflector assembly to concentrate the light through the liquid crystal display. A preferred embodiment can use at least two LEDs, or as many as six or more of each color, to provide the desired brightness level. Alternatively the LEDs can be arranged around the periphery of a transmissive display and directed down into a conical reflector that directs the backlighting through the display in concentrated form.
The backlight, the display and the viewing lens can be aligned along a single axis within a small housing volume that is less than 20 cm3, and preferably less than 12 cm3. The system weighs less than 10 grams, preferably in the range between 5 and 8 grams. The system can be incorporated into battery operated personal communication devices without substantial alteration of their form factor and weight requirements.
While a transmissive microdisplay with a backlight is preferred, a reflective microdisplay can also be used. The light from the light source is directed onto the same side of the display that is viewed by the user. An optical system directs the reflected image from the pixel electrodes onto a line of sight of the user. Reflective displays can be used in connection with the portable communications and display systems described herein.
The display can be operated using a color sequential system as described in U.S. patent application Ser. No. 08/216,817, xe2x80x9cColor Sequential Display Panelsxe2x80x9d filed on Mar. 23, 1994, which issued as U.S. Pat. No. 5,642,129, and of U.S. Pat. No. 5,673,059, the entire contents of these patents being incorporated herein by reference. These patents disclose an active matrix display in which the control electronics is integrated with the active matrix circuitry using single crystal silicon technology. The control electronics provides compressed video information to produce a color image for data, a still image or a video image such as a television image on the display. The use of LEDs to provide color sequential operation has a number of advantages. The system provides a lightweight, low-power light source that generates red, green and blue color components in sequence. The same control circuit operates the light source and the display to pulse the appropriate color elements for each corresponding display image.
The light source can also be pulsed for monochrome display applications. The same circuit can be used for both color sequential and monochrome systems. For monochrome operation the light source need only be flashed momentarily to provide the desired brightness level By flashing the lamp briefly while a given frame is written on the display, the display power consumption can be substantially reduced, the voltage holding requirements of the display are reduced, and heat loading is reduced. The vertical synchronization signal can be used to trigger the light source pulse which need only extend for less than a third of the time needed to write a particular frame onto the display. Two flashes in a frame can also be used to reduce flicker.
The microdisplays described herein can be used in head mounted displays, cameras, card readers and portable communications systems, including color sequential systems as described in greater detail in U.S. application Ser. No. 08/410,124 filed on Mar. 23, 1995, the entire contents of which is incorporated herein by reference. Further details regarding the drive electronics suitable for a microdisplay can be found in U.S. Ser. No. 08/106,416 filed on Aug. 13, 1993, the entire contents of which is incorporated herein by reference. A preferred embodiment of the display control circuit utilizes an xe2x80x9cunder scanningxe2x80x9d feature in which selected pixels are rapidly turned on and off to enhance edge definition and emulate a higher resolution display. The display control circuit can also utilize a panning capability so that a small portion of a displayed image can be selected, by mouse operation for example, and presented using the entire microdisplay image area thereby allowing the user to perceive smaller displayed features. This can also be used to view selected portions of a high resolution image, such as a portion of a 640xc3x97480 image on a 320xc3x97240 microdisplay.
As is readily apparent from the various embodiments described, one of the benefits of the microdisplay is the portability of the device using the microdisplay. An inherent concern with portability is providing enough power to operate the device for extended periods. One of the features of a preferred embodiment is the alternating of the voltage on the counterelectrode, therein allowing the microdisplay to operate at a lower voltage and therefore at a reduced power level. Another feature of a preferred embodiment is stopping the clock to the display when the display is not being written to, therein reducing power consumption.
When the display is used to display text, wherein the image display is not constantly changing, a feature of the preferred embodiment is to reduce the frame rate, or refresh rate. The reduction in frame rate results in a decrease in power consumption.
An additional problem with portability is the increased likelihood that the device will be used in non-ideal conditions. One such variable is the temperature in which the device will operate as temperature affects the performance of liquid crystal material. One of the features of a preferred embodiment is the monitoring of the temperature of the liquid crystal and the integral heating of the device.